Dermal interstitial fluid is a clear, water-like fluid present between the cells in the living skin tissues (i.e., epidermis and dermis) under the stratum corneum. The composition of the interstitial fluid is substantially free of blood cells, but does contain small molecules and proteins. Interstitial fluid both transports nutrients (e.g., glucose) from the blood to the skin cells and removes cellular metabolic wastes (e.g., urea) from the skin cells to the blood. Various components of the interstitial fluid are in equilibrium with living cells in both the epidermis and dermis as well as macromolecular structures located in the subcutaneous tissue.
The correlation of certain endogenous chemicals between the blood and interstitial fluid has been well established. For example, Service et al. used a novel minimally invasive technique to sample minuscule amounts (0.5 microliter) of the interstitial fluid from both healthy volunteers and diabetic patients. See Service et al., Diabetes Care, 20: 9, 1426-9 (1997). Service et al. assessed the accuracy of the glucose concentrations in the interstitial fluid in predicting concurrently measured venous plasma and capillary plasma glucose concentrations. Service et al. concluded that interstitial fluid sampling was a bloodless, minimally invasive technique that provided a medium for glucose measurement, the concentrations of which closely reflected ambient glycemia to a degree comparable with that of capillary glucose measurements. A similar conclusion was also drawn by Krogstad et al., British Journal of Dermatology, 134:6, 1005-12 (1996).
Others have also studied the concentration changes of certain drugs in the interstitial fluid following oral administration. See, e.g., Zimmerli et al., Antimicrobia. Agents. Chemother, 40:1, 102-4 (1996) and Vaillant et al., Eur. J. Clin. Pharmcol. 33:5, 529-30, (1987).
Although sampling interstitial fluid by minimally invasive techniques for diagnostic applications to monitor diseases such as diabetes is known, Applicant has found that certain endogenous substances present in the skin tissues (such as vitamin C) are at a different concentration in the interstitial fluid than in serum. Thus, analysis of substances in the serum is not indicative of the corresponding concentration levels of such substances in the skin. Also, Applicant has also found that the concentration of certain endogenous substance in the interstitial fluid varies as a result of diet, aging, or the effect of external aggressions. Applicant has discovered that the determination of the concentration of certain endogenous substances present in the interstitial fluid, thus, can be used to assess the health of a subjects skin, permitting the administration of cosmetic or pharmaceutical agents to specifically treat the needs of the subject's skin.